Democrats' latest litmus test is rejecting corporate PAC money. It's an easy way to appeal to their progressive, anti-establishment base that demands campaign finance reform.

By the numbers: More than 170 federal candidates have pledged not to take any donations from corporate PACS, AP's Lisa Lerer reports.

Between the lines: Lerer writes that this rejection "may be more symbolic than financial" because corporate donations really only make up a tiny fraction of political finance.