Appel, a former one-term state senator, has raised more than $500,000 for the year, her campaign told the Des Moines Register. She didn't say how much money she has in the bank.
The race for Latham's seat is a top priority for both parties. President Obama narrowly won the district in 2008 and 2012, but Latham handily beat fellow Rep. Leonard Boswell (D-Iowa) in the redrawn district in 2012. His retirement has created a top-tier opportunity for Democrats.
Republicans are worried a packed primary could hurt their candidates' fundraising and mean no candidate reaches 35 percent of the vote, automatically leading to a convention that could risk nominating a hard-right candidate.