Former President George H.W. Bush urged disgruntled conservatives on Monday to rally around John McCain, calling their criticism of the Republican presidential front-runner "grossly unfair."

The father of President George W. Bush said he was annoyed by attacks within the conservative wing of the Republican Party against the Arizona senator, the all-but-certain Republican nominee to face Democrats in November election.

Many conservatives distrust McCain because of his moderate views on illegal immigration and campaign finance reform and for having originally voted against President Bush's tax cuts. Persuading them all to vote for McCain in November will be a central challenge.

"His character was forged in the crucible of war. His commitment to America is beyond any doubt," the 41st U.S. president, flanked by his wife Barbara, told a joint news conference with McCain in a Houston airport hanger.

"You know, if you've been around the track you hear these criticisms and I think they are grossly unfair. He's got a ... sound conservative record but he's not above reaching out to the other side," he said.

"So I hear these criticisms and Barbara knows I get a little bit annoyed about them frankly," he said, calling them "absurd."