HOUSTON, Texas (CNN) -- Former President Bush endorsed John McCain on Monday and defended the conservative record of the Republican Party's presumptive presidential nominee.

Former President Bush endorses Sen. John McCain at an event in Houston, Texas, Monday.

Bush's endorsement was another sign the GOP's establishment is coalescing around the Arizona senator.

"At this critical time in history ... the United States of America cannot be allowed to falter," the 41st president said in Houston, Texas. "No one is better to lead our nation in these trying times than Senator John McCain.

"His character was forged in the crucible of war," Bush said, referring to McCain's experience as a Navy pilot and prisoner of war in Vietnam. Watch Bush praise McCain character »

Bush served as a Navy pilot during World War II. Like McCain, Bush was shot down in combat.

"Few men walking among us have sacrificed so much for the cause of human freedom," Bush said.

"I believe now is the right time for me to help John in his effort to start building the broad-based coalition it'll take for our conservative values to carry the White House this fall," the former president said.

After Bush spoke, McCain said that his endorsement would "help me enormously in the process of uniting our party and moving forward."

McCain is working to win the support the party's conservative base, many of whom were angered by the positions the Arizona Republican took on immigration, campaign finance and other issues as well as his willingness to work with Democrats to pass legislation.

Bush said the criticism of McCain's record by some conservatives was "absurd" and "grossly unfair."

"He's got a record that everybody can analyze in the Senate, a sound, conservative record, and yet he's not above reaching out to the other side," he said.

The former president, who served as President Reagan's vice president, noted that Reagan was attacked by "ultra-right" activists who said his record was not conservative enough when he ran for president in 1980.

McCain strategists see the Bush endorsement as a way to send another message to the former Arkansas governor Mike Huckabee to exit the race. The Bush nod also may bolster McCain in Texas, where a strong bloc of politically active social conservatives could embarrass him in the state's March 4 primary.

Though saying he "had not come here to tell any other candidate what to do," Bush seemed to suggest that it was time for Huckabee to end his campaign.

"After so much time and exhaustive effort by so many friends, it can take a while for any candidate to read the handwriting on the wall, and that certainly was true of me," he said.

After Monday's endorsement, McCain, who has 830 of the 1,191 delegates needed to win the nomination, heads to Wisconsin, where Huckabee, who has 217 delegates, is already campaigning.

On Thursday, former GOP rival Mitt Romney threw his support behind McCain, calling him a true American hero "capable of leading our country in this dangerous hour." Watch the two former rivals shake hands »

Romney had collected 286 delegates before he suspended his campaign two weeks ago. Those delegates would put McCain less than 100 delegates away from securing the nomination.

On Sunday, McCain brought memories of Bush's infamous broken promise -- "read my lips: no new taxes" -- after the 2008 candidate was asked whether he would make a similar pledge.

Speaking on ABC's "This Week," McCain said he would not increase taxes under any circumstances and mentioned several alternatives, including lowering interest rates and corporate tax rates "if our economy continues to deteriorate."

"There's a lot of things that I would think we should do to relieve that burden, including, obviously, as we all know, simplification of the tax code," he said.

Budget analysts, however, say it will be tough for McCain to keep his "no new taxes" pledge, like it was for former President Bush.

"The problems in the future are so large, that it's pretty unthinkable we could close those deficits either by just cutting programs or just raising taxes. We're going to have to do both sooner or later," said Robert Greenstein of the Center on Budget and Policy Priorities. E-mail to a friend

CNN's Dugald McConnell contributed to this report.

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