Key Republican Senate primaries in Missouri, Ohio, and Pennsylvania are poised for a shake-up after former President Donald Trump confirmed that endorsements are forthcoming in the three uncertain contests.

Trump is especially popular with the GOP base in those states, and the retirement of the incumbent Republican senator in each state sparked competitive primaries fought primarily over who is most closely aligned with the former president. Accordingly, with the candidates differing little on major issues, Trump’s endorsement could sway voters and effectively decide the winner of each campaign.