As presidential endorsements go, this one could hardly have been more low-key. ABC News caught up with former president George W. Bush in an elevator in downtown Washington on Tuesday and asked the question that elicited the sound bite.

“I’m for Mitt Romney,” Bush said, just as the doors slid shut. The 43rd president of the United States was on his way to give a speech on human freedom, in which he made no mention of politics, save one sidelong reference: “I actually found my freedom by leaving Washington.”

Bush’s absence from this year’s presidential race stands in sharp contrast with his predecessor, who has stayed in campaign mode pretty much since he left office.

In 2004, Bill Clinton appeared at a Philadelphia rally with Sen. John F. Kerry (Mass.), the Democratic nominee, only seven weeks after having quadruple bypass surgery. “If this isn’t good for my heart, I don’t know what is,” Clinton told a cheering throng of more than 100,000.

Despite the fact that his wife lost the Democratic nomination to Barack Obama in 2008, Clinton has eagerly stepped up as a validator and fundraiser for the current president in his bid for reelection. Then again, campaigning is such a part of Clinton’s DNA that he has even been willing to host fundraising events for state Senate candidates.

Don’t expect to see anything like that from Bush.

“We welcome the president’s support, as we welcomed his father’s” in late March, when former president George H.W. Bush bestowed his formal endorsement on the all-but-inevitable GOP nominee, said Romney campaign spokeswoman Andrea Saul.

“We’re proud to have the president’s support,” she added, “but he made clear when he left office that he was not going to engage in political campaigns and we have no reason to believe that is going to change.”

That the younger Bush has stayed invisible reflects both political reality — he remains far from popular among much of the electorate — and his sense of what his role should be as a former commander in chief.

Richard B. Cheney, his vice president, emerged as an early and vocal critic of the Obama presidency.

But Bush said of his successor in 2009: “I’m not going to spend my time criticizing him. There are plenty of critics in the arena. He deserves my silence. . . . I think it is essential that he be helped in office.”

It is far from clear whether it would help Romney if Bush took a more public role in a campaign in which the state of the economy is the top concern for voters. In a February poll by Quinnipiac University, 51 percent of respondents said Bush is more to blame for it, while only 35 percent said Obama is.

Meanwhile, many on the right have come to view Bush’s presidency as a time when the size of government and federal spending expanded — which is precisely the rap they are trying to pin on Obama now.

None of that is lost on the former president.

Bush “has a very grown-up view of who he is, and his reputation, and what the limits and power of his voice are,” said Tony Fratto, who was deputy press secretary in the Bush White House. “He knows that maybe, in some quarters, [his endorsement] is a benefit, and in some quarters, it is a liability.”

Meanwhile, the former president whose name Romney was invoking with approval at a campaign stop in Iowa on Tuesday was Clinton.

“Almost a generation ago, Bill Clinton announced that the era of big government was over,” Romney said. “President Obama tucked away the Clinton doctrine in his large drawer of discarded ideas, along with transparency and bipartisanship. It’s enough to make you wonder if maybe it was a personal beef with the Clintons.”