The good news for Weld and everyone involved: It's very unlikely that we'll be talking Wednesday morning about third-party spoilers — even if it is a close race.
The fact is that the presence of Libertarian presidential nominee Gary Johnson on the ballot in all 50 states and Green Party nominee Jill Stein in most states doesn't seem to be altering the race in any demonstrable way. The combination of the two of them, in fact, seems to be affecting Clinton and Trump almost equally. (About the only logical way in which a third-party candidate could affect the election outcome would be if Trump somehow lost Utah to either third-party candidate Evan McMullin, or to Clinton because McMullin took enough of the Republican vote. But that would require Utah's six electoral votes to be pivotal.)
We ran the Real Clear Politics averages in key states for the two-way matchup (Trump vs. Clinton) and compared it to the three- or four-way matchups that actually exist in those states. In many states, the leader's margin was completely unchanged. In a few states, it nominally favored Trump, and in a few others, it favored Clinton.
The only state in which it changed the margin by more than 0.3 points was Ohio, which went from a 2.2-point Trump margin without third-partiers to a 3.5-point Trump margin with them included. But this is almost completely because a Columbus Dispatch poll that showed Clinton with a rare lead in the state didn't test a race with third-party candidates.
There was plenty of speculation at the outset that Johnson's candidacy could eat into Trump's voter base, because Libertarians tend to be closer to the Republican Party's overall philosophy and Trump was broadly unpopular. But Johnson also appears to have eaten into Clinton's numbers. You add in Stein's 1 to 2 percent support in most polls — probably mostly at Clinton's expense — and the overall effect on the race is very balanced.
Of course, it doesn't take a big chunk of the vote to technically change an election. In 2000 in Florida, just 537 votes out of 6 million cast handed the state and the electoral college victory to Bush. Nader took more than 97,000 votes statewide, so it's very likely that Gore would have won Florida and the presidency if Nader hadn't run. (Reform Party nominee Pat Buchanan, who had run previously as a Republican, also could be accused of taking votes from Bush, but he won far fewer votes — about 17,500 — than Nader).
So if we have a nail-biter tonight and it comes down to one or two closely decided states, you could make an argument that Johnson and/or Stein not being in the race might have changed things. And if Trump is on the short end of a close result, you can bet he'll add third-party spoilers to his claims of a rigged electoral system.
But even if Johnson and Stein take historically high percentages of the vote for their parties, it doesn't appear at all likely that this will have fundamentally changed the result.