AMES, Iowa - US Democratic presidential candidate Hillary Clinton on Tuesday expanded her plan to tax the wealthiest Americans by saying she would close tax-avoidance "loopholes" that allow investments to be routed through low-tax countries and put in tax-deferred retirement accounts."My plan is kind of simple: we go after the wealthy to pay for what the middle-class working people and poor people need," Clinton said at a campaign stop in Ames, Iowa on Tuesday."I think that's a smart way to help fund some of our priorities," Clinton added.Clinton said she would end the "Bermuda reinsurance loophole" that allows hedge fund managers to route investments through an insurance company in a low-tax country such as Bermuda to avoid paying a higher tax rate.Clinton also said she would close what she calls the "Romney loophole," which allows wealthy individuals to shield large sums of money in individual retirement accounts from taxation.Her proposals come after a plan announced Monday to add a 4.0 percent income-tax surcharge on earners of more than $5 million per year.Clinton has reiterated her support for the Buffett rule, named after billionaire investor and Clinton endorser Warren Buffett, which would institute a minimum tax rate of 30 percent on those earning more than $1 million per year.Clinton, long the Democratic party's front-runner for the nomination, is unveiling her plan to tax the wealthy amid slipping poll numbers in the key early-voting states of Iowa and New Hampshire as her chief rival, U.S. Senator Bernie Sanders from Vermont, has wooed supporters with his plan to address income inequality.On Tuesday, a Quinnipiac poll saw Sanders win a five-point lead over Clinton among likely Iowa caucus-goers. In New Hampshire, Sanders outpaced the former secretary of state by 14 points, securing 53 percent of the party's support, a Monmouth University survey said. The polls had an error margin of 4.4 and 4.8 points, respectively.Iowa voters will kick off the nation's nominating race with caucuses held on Feb. 1, and New Hampshire voters will cast ballots in the first-in-the-nation primaries on Feb. 9.