Democratic presidential candidate Hillary Clinton on Wednesday will unveil proposals to deter U.S. companies from shifting profits overseas, including an "exit tax" to penalize companies that perform so-called tax inversions, a campaign official said.

The exit tax would apply to companies like Pfizer that move abroad for tax advantages, said the official, who didn't want to be named ahead of the official announcement. Another major part of Clinton's plan will be announced Wednesday, when Clinton is due to appear in the first-in-the-nation caucus state of Iowa, the official said. 

Clinton will also restate her support for raising, to 50 percent from 20 percent, the threshold for shares a U.S. company can transfer to a foreign owner to gain tax benefits, said the official. The Obama administration also backs raising the threshold.