The president is elected by votes from the electoral college where there are 538 votes.
Votes are based 435 congressional districts (apportioned to states based on population) plus 100 Senators (2 per state) plus 3 votes for DC. You need 270 votes to win.
Most states have a winner take all electoral vote system. Maine and Nebraska, use a tiered system where a single elector is chosen within each Congressional district and two electors are chosen by statewide popular vote.
See Electoral College (United States) - Wikipedia

This system allows a candidate to loose the popular vote (As George W. Bush did in 2000 (Bush - 50,456,002 votes, Gore - 50,999,897 votes), but win the election.

The other seeming inequity in the system is if you are in a state that strongly supports one party or the other, your vote doesn't matter because the state is always going to going to cast all their electoral votes for the winner.

This is true for the following in 1992-2012;
Republican States: Alabama, Alaska, Idaho, Kansas, Mississippi, Nebraska, North Dakota, Oklahoma, South Carolina, South Dakota, Texas, Utah, Wyoming.

and Democratic States: California, Connecticut, Delaware, D.C., Hawaii , Illinois, Maine, Maryland, Massachusetts, New Jersey, New York, Michigan, Minnesota, Oregon, Pennsylvania, Vermont, Washington, Wisconsin.
Arkansas and Tennessee had the native son effect with Clinton and Gore in '92 and '96, or they would have been on this list.

The exceptions are Reagan's popularity in the 1980's and Roosevelt's popularity in the 30's.

The 1960's marked a shift in political ideologies.
Southern conservatives where Democrats because they didn't want to belong to the party of Lincoln after the slavery issue in the Civil war. They decided it was OK to be Republican after Barry Goldwater, who had voted against the Civil Rights act became the Republican candidate in 1964.
Another thing pushing southern conservatives was voter registration efforts in the 60's, which brought more African Americans into the Democratic party.
These two issues accelerated the transition from a solid South for the Democrats to one for the Republicans.

In 1967 George Wallace ran for the American Independent party and got 13.5% of the vote.

Swing States and Safe States:
Safe states - republican: 17 states - 138 Electoral College Votes (ECV)
Safe states - democratic: 15 states - 186 ECV
Swing states: 19 states - 214 ECV (States which have been swing states in at least one of the last 5 elections)
FiveThirtyEight lists 7 swing states for 2016, Wikipedia and Politico listed 11 states for 2012 (but 2 are different), CNN listed 7 battleground states for 2008, The Wall Street Journal listed 16 battleground states for 2000.

Candidates																			Safe states and swing states
State	1948	1952	1956	1960	1964	1968	1972	1976	1980	1984	1988	1992	1996	2000	2004	2008	2012	2016	State	ECV
Winner ECV *	303	442	457	303	486	301	520	297	489	525	426	370	379	271	286	365	332	279	Swing states SS
Winner Popular %	49.6	54.9	57.4	49.7	61.1	43.4 1	60.2	50.0	50.5	58.8	53.4	43.0 1	49.2 1	47.9 1	50.7	52.9	51.1	47.2 1
Alabama	SR	D	D	I	R	AI	R	D	R	R	R	R	R	R	R	R	R		AL	9
Alaska				R	D	R	R	R	R	R	R	R	R	R	R	R	R		AK	3
Arizona	D	R	R	R	R	R	R	R	R	R	R	R	D	R	R	R	R		AZ	11
Arkansas	D	D	D	D	D	AI	R	D	R	R	R	D2	D2	R	R	R	R		AR	6
California	D	R	R	R	D	R	R	R	R	R	R	D	D	D	D	D	D		CA	55
Colorado	D	R	R	R	D	R	R	R	R	R	R	D	R	R	R	D	D		CO	9
Connecticut	R	R	R	D	D	D	R	R	R	R	R	D	D	D	D	D	D		CT	7
Delaware	R	R	R	D	D	R	R	D	R	R	R	D	D	D	D	D	D		DE	3
D.C.					D	D	D	D	D	D	D	D	D	D	D	D	D		DC	3
Florida	D	R	R	R	D	R	R	D	R	R	R	R	D	R	R	D	D		FL	29
Georgia	D	D	D	D	R	AI	R	D	D	R	R	D	R	R	R	R	R		GA	16
Hawaii				D	D	D	R	D	D	R	D	D	D	D	D	D	D		HI	4
Idaho	D	R	R	R	D	R	R	R	R	R	R	R	R	R	R	R	R		ID	4
Illinois	D	R	R	D	D	R	R	R	R	R	R	D	D	D	D	D	D		IL	20
Indiana	R	R	R	R	D	R	R	R	R	R	R	R	R	R	R	D	R		IN	11
Iowa	D	R	R	R	D	R	R	R	R	R	D	D	D	D	R	D	D		IA	6
Kansas	R	R	R	R	D	R	R	R	R	R	R	R	R	R	R	R	R		KS	6
Kentucky	D	D	R	R	D	R	R	D	R	R	R	D	D	R	R	R	R		KY	8
Louisiana	SR	D	R	D	R	AI	R	D	R	R	R	D	D	R	R	R	R		LA	8
Maine	R	R	R	R	D	D	R	R	R	R	R	D	D	D	D	D	D		ME	4
Maryland	R	R	R	D	D	D	R	D	D	R	R	D	D	D	D	D	D		MD	10
Massachusetts	D	R	R	D	D	D	D	D	R	R	D	D	D	D	D	D	D		MA	11
Michigan	R	R	R	D	D	D	R	R	R	R	R	D	D	D	D	D	D		MI	16
Minnesota	D	R	R	D	D	D	R	D	D	D	D	D	D	D	D	D	D		MN	10
Mississippi	SR	D	D	I	R	AI	R	D	R	R	R	R	R	R	R	R	R		MS	6
State	1948	1952	1956	1960	1964	1968	1972	1976	1980	1984	1988	1992	1996	2000	2004	2008	2012	State	ECV
Missouri	D	R	D	D	D	R	R	D	R	R	R	D	D	R	R	R	R		MO	10
Montana	D	R	R	R	D	R	R	R	R	R	R	D	R	R	R	R	R		MT	3
Nebraska	R	R	R	R	D	R	R	R	R	R	R	R	R	R	R	R	R		NE	5
Nevada	D	R	R	D	D	R	R	R	R	R	R	D	D	R	R	D	D		NV	6
New Hampshire	R	R	R	R	D	R	R	R	R	R	R	D	D	R	D	D	D		NH	4
New Jersey	R	R	R	D	D	R	R	R	R	R	R	D	D	D	D	D	D		NJ	14
New Mexico	D	R	R	D	D	R	R	R	R	R	R	D	D	D	R	D	D		NM	5
New York	R	R	R	D	D	D	R	D	R	R	D	D	D	D	D	D	D		NY	29
North Carolina	D	D	D	D	D	R	R	D	R	R	R	R	R	R	R	D	R		NC	15
North Dakota	R	R	R	R	D	R	R	R	R	R	R	R	R	R	R	R	R		ND	3
Ohio	D	R	R	R	D	R	R	D	R	R	R	D	D	R	R	D	D		OH	18
Oklahoma	D	R	R	R	D	R	R	R	R	R	R	R	R	R	R	R	R		OK	7
Oregon	R	R	R	R	D	R	R	R	R	R	D	D	D	D	D	D	D		OR	7
Pennsylvania	R	R	R	D	D	D	R	D	R	R	R	D	D	D	D	D	D		PA	20
Rhode Island	D	R	R	D	D	D	R	D	D	R	D	D	D	D	D	D	D		RI	4
South Carolina	SR	D	D	D	R	R	R	D	R	R	R	R	R	R	R	R	R		SC	9
South Dakota	R	R	R	R	D	R	R	R	R	R	R	R	R	R	R	R	R		SD	3
Tennessee	D	R	R	R	D	R	R	D	R	R	R	D2	D2	R	R	R	R		TN	11
Texas	D	R	R	D	D	D	R	D	R	R	R	R	R	R	R	R	R		TX	38
Utah	D	R	R	R	D	R	R	R	R	R	R	R	R	R	R	R	R		UT	6
Vermont	R	R	R	R	D	R	R	R	R	R	R	D	D	D	D	D	D		VT	3
Virginia	D	R	R	R	D	R	R	R	R	R	R	R	R	R	R	D	D		VA	13
Washington	D	R	R	R	D	D	R	R	R	R	D	D	D	D	D	D	D		WA	12
West Virginia	D	D	R	D	D	D	R	D	D	R	D	D	D	R	R	R	R		WV	5
Wisconsin	D	R	R	R	D	R	R	D	R	R	D	D	D	D	D	D	D		WI	10
Wyoming	D	R	R	R	D	R	R	R	R	R	R	R	R	R	R	R	R		WY	3
State	1948	1952	1956	1960	1964	1968	1972	1976	1980	1984	1988	1992	1996	2000	2004	2008	2012	State

AI - American Independent a far right party created by Bill and Eileen Shearer, (Eileen was the daughter of California senator Wiliam Knowland). George Wallace, former Alabama governor, ran as an AI in 1968.
I - Independent
SR = States' Rights - A far right, white supremacist party.
ECV = Electoral College Votes

* Prior to 1960 there were only 531 electoral votes and you needed 266 to win.

1. In 1968 George Wallace got 13.5% of the vote as an American Independent.
In 1992 Ross Perot got almost 19% of the vote as an independent.
In 1996 Perot got 8.4%.
In 2000 Ralph Nader got 2.7%, Gore, the looser in ECV, got 48.4%, Bush got 47.9% of the popular vote.
In 2016 Libertarian candidate Gary Johnson got 3% of the vote. Hillary Clinton, who lost, got 48.7% of the popular vote.

2. In 1992 and '96 Arkansas and Tennessee went republican for favorite sons Clinton and Gore

Sources:
List of United States presidential election results by state - Wikipedia
Popular Votes 1940-2012 - Roper Center

See Margin of victory - Republican & Democratic States in 2000 the closest election.

I had a housemate in grad school who was doing his PhD thesis on the fairness of the electoral system. He came to the conclusion that it was the best system. I don't remember the criteria he used to judge the the different options.

The 11 states with the largest population account for more than half of EC Votes.

Battleground States

Safe states - Republican: 13 states - 102 Electoral College Votes (ECV)
Safe states - Democratic: 19 states - 242 ECV (Includes DC)
Swing states:             19 states - 194 ECV

Note: The table to the left counts Wisconsin, Minnesota, Pennsylvania, Oregon and Washington as safe democratic states, but they have been considered battleground states at times by the compaigns.

X - Battleground State

State		2000	2004	2008	2012	2016
				Here	WSJ	CNN	Wiki- pedia	Politico
FL	Florida	x	x	x	x	x
OH	Ohio	x	x	x	x	x
IA	Iowa	x	x		x	x
NV	Nevada	x	x		x	x
NH	New Hampshire	x	x		x	x
PA	Pennsylvania	x	x		x	x
WI	Wisconsin	x	x		x	x
MN	Minnesota	x	x		x	x
MI	Michigan	x	x			x
NC	North Carolina			x	x	x
MO	Missouri	x	x	x
CO	Colorado				x	x
VA	Virginia				x	x
AZ	Arizona	x				x
WV	West Virginia	x	x
AR	Arkansas	x	x
NM	New Mexico	x	x
OR	Oregon	x	x
WA	Washington	x	x
GE	Georgia					x
IN	Indiana			x
MT	Montana			x
ND	North Dakota			x
ME	Maine	x

Swing state - Wikipedia

The 11 states that will determine the 2016 election - POLITICO

Tipping-Point (or battleground) states (Oct 12, 2016) per fivethirtyeight.com:
State       Chance      2012       2008       2004

Florida        16.6%  Obama  50%  Obama 51%  Bush  52%
Pennsylvania   11.5   Obama  52%  Obama 55%  Kerry 51%
North Carolina  9.2   Romney 51%  Obama 50%  Bush  56%
Colorado        7.9   Obama  51%  Obama 54%  Bush  52%
Ohio            7.3   Obama  50%  Obama 52%  Bush  51%
Virginia        7.2   Obama  51%  Obama 53%  Bush  54%
Wisconsin       6.8   Obama  53%  Obama 57%  Kerry 50%
Minnesota       5.6   Obama  53%  Obama 55%  Kerry 52%
Michigan        5.4   Obama  54%  Obama 58%  Kerry 52%

Democrat			Republican
State		ECV	Margin of victory
			Clinton		Bush		Obama
			1992	1996	2000	2004	2008
National			5.6%	8.5%	-0.5%	2.8%	6.5%
Florida	FL	27	1.89%	5.70%	0.0%	5.0%	3%
Missouri	MO	11	10.15%	6.30%	3.3%	7.2%	0%
Ohio	OH	20	1.83%	6.36%	3.5%	2.1%	4%
Iowa	IA	7	6.02%	10.34%	0.3%	0.7%	9%
New Hampshire	NH	4	1.22%	9.95%	1.3%	1.4%	9%
North Carolina	NC	15	0.79%	4.69%	12.8%	12.4%	0%
Virginia	VA	13	4.38%	1.95%	8.0%	8.2%	7%
New Mexico	NM	5	8.56%	7.32%	0.1%	0.8%	15%
Pennsylvania	PA	21	9.02%	9.20%	4.2%	2.5%	11%
Arizona	AZ	10	6.77%	2.23%	6.3%	10.5%	9%
Nevada	NV	5	2.63%	1.02%	3.5%	2.6%	12%
Colorado	CO	9	4.26%	1.37%	8.4%	4.7%	9%
West Virginia	WV	5	13.02%	14.75%	6.3%	12.9%	13%
Michigan	MI	17	7.39%	13.21%	5.1%	3.4%	16%
Montana	MT	3	2.51%	2.88%	25.1%	20.5%	3%
Indiana	IN	11	6.12%	5.58%	15.6%	20.7%	1%
Wisconsin	WI	10	4.35%	10.33%	0.2%	0.4%	14%
Minnesota	MN	10	11.63%	16.14%	2.4%	3.5%	10%
Tennessee *	TN	11	4.65%	2.41%	3.9%	14.3%	15%

ECV - Elctoral College votes
* Arkansas and Tennessee had the advantage of the native son effect with Clinton and Gore in '92 and '96.

See:
Presidential Battleground States: A History

Forecasting References:
Election forecasting in the age of Trump - LA Times

Nate Silver, statistician, baseball player analyst, FiveThirtyEight Blog, successfully called the outcomes in 49 of the 50 states in the 2008 U.S. Presidential election.

Fundamental Models for Forecasting Elections

Source: Was Nate Silver the Most Accurate 2012 Election Pundit? - CFAR (Center for Applied Rationality)

Links:
List of United States presidential election results by state - Wikipedia
U.S. Balance of power President - Congress
www.PresidentElectionPolls.com/
Presidential Elections at Infoplease
US Presidential Election Maps: 1860-1996 at U. Virginia
County Voting Patterns in US Presidential Elections, 1976-2000
Maps at Washington Post
Presidential Elections 1789-2000 Results by county 2000 & 2004 at USA TODAY
Red States feed, Blue states Supply at TaxProf Blog
Presidential Battleground States: A History | The Federalist
List of United States presidential elections by popular vote margin - Wikipedia

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