For Al Gore and George W. Bush, the McCain vote has become the holy grail of the presidential race, the swing vote each man thinks he needs to put him over the top. But this line of reasoning has one big problem: there is no “McCain vote” — the exit data from the primaries show that John McCain’s supporters are not the sort of portable voting bloc than can be won en masse.

Across the country, McCain backers do not share values or care strongly about the same issues, and they are not drawn from a common demographic base.

While political reform was the keystone of the Arizona senator’s campaign, only a minority of McCain voters cited campaign finance changes as the foremost issue. In New York, only 19 percent of his backers made this claim — more (23 percent) said “moral values” was their top concern — and the number was no bigger in other Super Tuesday states including California (14 percent), Ohio (14 percent), Maryland (18 percent) and Massachusetts (18 percent). Even in Mr. McCain’s greatest triumphs, the New Hampshire and Michigan primaries, only minorities of his backers were reform-minded.

In broader questioning in a nationwide survey that Pew conducted in February, Republicans and independents who backed Mr. McCain were indistinguishable from Bush voters when asked about the budget surplus: 37 percent of McCain voters and 39 percent of Bush backers wanted it used to shore up entitlements. Yes, fewer McCain voters than Bush supporters also emphasized a tax cut — 11 percent to 24 percent. But the two constituencies were nearly identical in their beliefs that the way officials campaign for office is a much smaller problem than the way they will govern or their basic honesty and ethics.

Likewise, McCain backers share few demographic bonds. While men are somewhat more drawn to him than women, his constituency has no decided socioeconomic pattern or ethno-religious coloration. Mr. McCain did poorly among one core Republican group, Christian conservatives. Exit polls in New York and California even found Mr. Bush carrying the Roman Catholic vote. Nor was there the expected veterans’ brigade. Mr. McCain carried the veterans’ vote in states that he won, like Michigan and New Hampshire, but lost it in New York, Ohio and California.

What stands out consistently about the McCain bloc is that it was drawn to all of the things the senator personally represents. Nearly half of the McCain voters in New York said they were looking for a candidate who stands up for what he believes — a view shared by just 26 percent of the Bush crowd. Similarly, in just about every exit poll, the McCain backers put more emphasis than other voters on a candidate’s personal qualities as opposed to his stand on issues.

The two candidates left standing don’t seem to be paying these numbers any heed. Mr. Gore, heartened by Super Tuesday exit polls showing that up to 40 percent of McCain supporters might vote for him in the fall, quickly raised the banner of campaign finance reform in a blatant attempt to woo them. The Bush camp hasn’t ruled out a reform plan of its own as it weighs strategies to get Mr. McCain on board.

The problem is, neither Al Gore nor George Bush can target McCain voters and say, “Vote for me — I am more like McCain than my opponent.” Authenticity is not seen as the strong suit of either man.

This is not to say that the independent vote will not be crucial in the fall. But independents are the swing voters in every election. And no evidence exists that John McCain has fundamentally changed the views, priorities or values of independents.

The points of contention among independents will likely be Mr. Gore’s perceived character weaknesses versus the lingering doubts about Mr. Bush’s depth, as well as voters’ displeasure with the Republican Congress and the party’s weaknesses on leading issues. Polling suggests that Catholics, older voters and young women are having a particularly hard time choosing between the two men.

There is no reason to think that embracing any of John McCain’s positions will do much for either candidate. Nor is it certain that his endorsement would help Mr. Bush all that much. In the end, John McCain was a candidate, not a cause.