Corporations have begun to send a majority of donations from their political action committees to Republican candidates, a reversal from the trend of the past three years.

See donations to Republican and Democratic candidates from corporations' political action committees.

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The change in corporate PAC giving is the latest sign Republicans are likely to make significant gains in November's midterm elections. Business PACs are notoriously cautious in deciding which party should receive a majority of their donations. They nearly always give most of their contributions to candidates whose political party is in power on Capitol Hill.

According to data from the Center for Responsive Politics, business PACs gave 52% of their $72.2 million in total...