The historical record supports your claim. Nader's third party candidacy drew off enough votes to bring about Al Gore's defeat in 2000. The Perot voters in 1992 and 1996 would have probably broken 2:1 in favor of the elder Bush and Bob Dole. Had that occurred, Clinton might have been defeated.
This country has not had a successful new party since 1856, when the Republicans replaced the Whigs as a successful opposition party. However, third parties do have an ideological impact. The People's (Populist) Party in the 1890s and the Progressive and Socialist Parties in the early 1900s drew from the same base of small farmers and urban workers that had been the Democrats' base since the days of Thomas Jefferson. In response, the Democrats shifted from being a party of weak Federal power, sound money, and low taxes to becoming a centralist, inflationary, and tax and spend party. Grover Cleveland, the last Jeffersonian President, was more conservative than Calvin Coolidge. A mere 40 years later, Franklin Roosevelt was our first modern liberal President.
The threat of a third party on the right flank could have the effect of restraining a Republican lurch to the left. Alan Keyes or Tom Tancredo may never win, but if they have ballot position in 50 states and a decent war chest, they could easily draw 5% of the vote in a race between Hillary and a RINO. Given the tightness of the last four Presidential elections, 5% would be more than enough to ensure a Democrat victory. The GOP leadership no doubt realizes this, and will hopefully take measures, other than screaming "sit down and shut up" at the conservatives.
What is more disconcerting about our party is in an article in our paper this morning (from a WaPO editorial) which bears out the poll Neal Boortz quoted, that the libertarian arm of the GOP is growing and at the same time becoming disenchanted with the conservative wing of the party... they also feel they are larger and don't have to bend to the conservatives any longer. It went on to point out the GOP has been virtually eliminated in many Northern states and is fast losing ground in the West and parts of the South.