Less than four weeks before Americans are scheduled to elect their next president, the race is anything but over. Incumbent president Barack Obama is virtually tied with his Republican challenger Mitt Romney in preelection polls. There is a chance that neither candidate secures the majority they need to win.

Americans elect their president and vice president not by popular vote but through an electoral college system that advantages smaller states. Nevertheless, the outcome of the popular vote hardly ever differs from the outcome in the Electoral College. The most recent exception was in 2000 when Democrat Al Gore won roughly half a million more votes nationwide but George W. Bush won five more points in the Electoral College.

With 538 Electoral College votes up for grabs, it is possible that neither candidate wins the 270 that are needed to win.

NBC News’ Chuck Todd explored three scenarios for an Electoral College tie on The Daily Rundown on Thursday.

In the first, the president wins Colorado, Virginia and Wisconsin while Romney wins a majority in all the other of the nine battleground states — the least likely scenario given Virginia’s traditional Republican preference.

In the second, the president wins New Hampshire, Ohio and Wisconsin — more likely as these three states tend to favor Democrats.

Recent polls in New Hampshire suggest that Romney could win there, however, while the president has consistently polled ahead in Nevada. A third scenario gives Obama the states Colorado, Ohio and Nevada but splits the Electoral College votes in Maine, a largely Democratic state that could award one of its electoral votes to the Republican candidate.

Maine is one of two states that splits its Electoral College votes. The other is Nebraska. It first split its votes in 2008 when Republican John McCain won a statewide victory there but Barack Obama won one electoral vote.

An Electoral College tie only once occurred before. In 1800, both Thomas Jefferson and his vice presidential candidate Aaron Burr won 73 electoral votes. They were nominated by the same party but ballots did not distinguish between presidential and vice presidential votes at the time. The election was ultimately decided in the House of Representatives.

That could happen again if neither candidate emerges with an Electoral College majority in November. The lower chamber of Congress elects the president but the Senate elects the vice president. Because Republicans are likely to retain their majority in the House while Democrats could well hold on to their majority in the Senate, such a procedure could see Mitt Romney elected president and Joe Biden remaining vice president.

Before a vote comes to Congress, the electors who are actually appointed on November 6 meet to validate the election results on December 17. They are not bound to vote for the winner of the popular vote in all states. If there is a tie, one “faithless elector” in Iowa or New Hampshire could sway the election in either candidate’s favor. The last time an elector voted for another party than he was supposed to was in 1972.

No such “faithless” electoral vote has ever changed the outcome of a presidential election. The last time the electoral college failed to deliver a majority for a vice presidential candidate was in 1836 when the Virginia delegation refused to vote for Richard Mentor Johnson. He was subsequently elected in the Senate.

Based on opinion polls, it is likelier that either Obama or Romney secures a slim Electoral College majority than neither candidate winning 270 votes or more. If it does happen, it could be weeks or months — Congress doesn’t convene until January — before Americans know who their president will be for the next four years.