(AP) -- Hillary Rodham Clinton made no mention of her potential Republican rivals when kicking off her second campaign for president.

They didn't share her restraint.

Rand Paul put Clinton at the center of his first television ad, titled "Liberty, not Hillary." On Twitter, Scott Walker fired off a series of messages describing Clinton as having a "Washington-knows-best mentality." Jeb Bush derided the "Obama-Clinton foreign policy" in a video message.

Wrote South Carolina Sen. Lindsey Graham on Twitter, "The middle class is getting screwed by the administration's domestic agenda & I believe it would be more of the same with Clinton."

While there are serious policy differences among the dozen or so major Republicans considering a run for president, they appear to have all concluded there's little downside in starting early when it comes to going after Clinton -- an opponent that only one of them has the chance to potentially face.

Even former Massachusetts Gov. Mitt Romney, the party's 2012 nominee, couldn't resist taking a shot at Clinton on Sunday.

"Hillary Clinton is just not trustworthy," he said on "Fox News Sunday." ''She's a creature of Washington."

Meantime, she hit the road -- quite literally.

Clinton left Sunday on a road trip from her home in Chappaqua, New York, in a van headed for Iowa, home of the nation's first presidential caucuses.

The former secretary of state will hold her first campaign event on Tuesday in eastern Iowa. The road trip was Clinton's idea, aides said.

Longtime aide Huma Abedin, in a conference call with Clinton alumni, said that Clinton's van stopped at a gas station in Pennsylvania, where the former first lady and New York senator met a family from Michigan.

A Clinton aide said the van is nicknamed "Scooby" after the Mystery Machine van in the 1970s animated television show, "The Scooby Doo Show." The aide said Clinton was a passenger and the van was driven by the Secret Service. The trip will cover roughly 1,000 miles between Clinton's home in New York City's suburbs and Iowa. Clinton expects to arrive in the state on Monday and is appearing at a community college in Monticello, Iowa, on Tuesday.

In the hours before she formally entered the race in a video posted to her website on Sunday, Clinton was subject to withering criticism from Republicans such as Romney who showed up on the Sunday morning political talk shows. In the hours that followed, there was more of the same -- with the added request for donations.

Bush hit up supporters with a fundraising appeal to help him stop Clinton's "liberal agenda." Paul started selling "Hillary's Hard Drive" on his website, a not-so-subtle reference to Clinton's use of a personal email account and server while serving as secretary of state.

The salvos at Clinton come amid an already-underway effort by the Republican National Committee and a series of conservative groups to paint Clinton in a negative light. At least 10 political organizations list defeating Clinton as their primary mission in their filings with the Federal Election Commission, and Democrats say they're preparing to face much as $500 million worth of attack ads during the 2016 election.

While the early efforts aim to highlight Clinton's potential vulnerabilities, Republicans also see these initial emails, videos and tweets as an opportunity to collect valuable email addresses and other data about potential supporters.

"Ninety-eight percent of our focus has been on Hillary for the last year," said Sean Spicer, the chief strategist and communications director of the Republican National Committee. "And having all this digital political and communication assets focused on Hillary gives us a massive advantage. It's one thing to motivate your base, it's another thing to activate them."

Republican strategists say the Clinton-focused messages appeal to primary voters by demonstrating a candidate's ability to take her on in the general election.

Florida Sen. Marco Rubio's team was initially concerned that coverage of Clinton's announcement would crowd out coverage of their splashy Monday night rally that's expected to kick off his 2016 campaign. They later concluded the almost overlapping events would draw a powerful contrast between Rubio, the 43-year-old senator, and Clinton, who is almost 25 years his senior.

It's a calculus that didn't occur, or didn't matter, to Clinton. Her video, shared hundreds of thousands of times in the first hour after being posted online, said nothing about anyone else in the 2016 race.

An old Clinton rival from Arkansas who is also considering a run for president, former Gov. Mike Huckabee, predicted that silence won't last.

"Anyone who thinks that she's going to get into this halfheartedly, well, they've never ever encountered the will, the spirit, the heart and the determination of the Clinton political machinery," he said on ABC's "This Week."

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Associated Press writer Philip Elliott contributed to this report from Washington.

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Follow Lisa Lerer on Twitter at: http://twitter.com/llerer