On Oct. 12, 1948, the campaign train of Tom Dewey, the Republican nominee against President Harry Truman, pulled into Beaucoup, Ill., where, from the rear platform, he would speak to about 1,000 people. Before he began, the engineer mistakenly caused the train to lurch a few feet backward, frightening some but injuring none.

Dewey, however, hurt himself by angrily saying into the microphone, “That’s the first lunatic I’ve had for an engineer. He probably ought to be shot at sunrise.” Dewey’s “cold arrogance” (Truman biographer David McCullough’s description) reinforced the public’s impression of an unsympathetic and prickly politician.

Truman ran against a Republican-controlled Congress but won because Dewey was off-putting. And Truman won despite two splinter candidacies from his party — those of former vice president Henry Wallace on the left and South Carolina Gov. Strom Thurmond on the right. Each won 2.4 percent of the popular vote.

Because Thurmond’s support was regionally concentrated, he won 39 southern electoral votes. If Truman had lost two of three states — Ohio, Illinois and California (he won them by just 7,000 votes, 34,000 and 18,000, respectively) — no candidate would have won an electoral vote majority, and the House of Representatives would have picked the president. If Dewey had won all three, he would have been president.

So, small vote totals for independent candidacies can have huge potential consequences. Which brings us to Ron Paul.

When recently asked if he might mount an independent candidacy, he said: “I’m not thinking about it because, look, I’m not doing badly right now. . . . So we concentrate only on one thing: Keep moving up in the polls, and see how things come out in a month or two.”

He is in the top tier in Iowa and would alienate Republican voters if he indicated an interest in bolting the party next autumn. Nationally, his ceiling is low, but his floor is solid: His supporters are inclined to accept no substitutes because no other candidate espouses anything like his high-octane blend of libertarianism and isolationism.

Furthermore, he is now nationally known (he campaigned for the 2008 Republican nomination and was the Libertarian Party’s 1988 presidential candidate) and has a large base of small donors. His intense supporters probably could get his name on most states’ ballots. He is not seeking reelection to his House seat, so what has he got to lose?

Well, his candidacy might guarantee Barack Obama’s reelection, and this might hurt the career of his son Rand, the freshman senator from Kentucky. Other than that, however, Ron Paul may think what his ideology implies — that Obama is only marginally more mistaken than Paul’s Republican rivals, who do not wake up each day angry about the 1913 Federal Reserve Act.

So, assume three things. That Obama is weaker in 2012 than he was when winning just 53 percent of the vote in 2008. That Paul could win between 5 percent and 7 percent of the vote nationally (much less than the 18 percent that a recent NBC-Wall Street Journal poll showed were prepared to vote for Paul as an independent). And that at least 80 percent of Paul’s votes would come at the expense of the Republican nominee.

Based on states’ results in 2000, 2004 and 2008, and on states’ previous votes for third-party candidates, and on current polling about the strength of potential Republican nominees in particular states, it is plausible to conclude that a Paul candidacy would have these consequences:

●It would enable Obama to carry two states he lost in 2008: Missouri (11 electoral votes), which he lost by 0.13 points, and Arizona (11), which he lost by 8.52 points to native son John McCain.

●It would enable Obama to again win four states he captured in 2008 and that the Republican nominee probably must win in 2012: Florida (29), Indiana (11), North Carolina (15) and Virginia (13).

●It would secure Obama’s hold on the following states he won in 2008 but that Republicans hope to take back next year: New Mexico (5), Colorado (9), Nevada (6), Michigan (16), Ohio (18), Pennsylvania (20) and New Hampshire (4).

At a minimum, a Paul candidacy would force the Republican nominee to spend time and money in places he otherwise might be able to economize both. And a Paul candidacy would make 2012 much easier for Obama than 2008 was. Now, reread Paul’s words quoted above, particularly these: “right now” and “in a month or two.”