Jeb Bush is entering a critical phase of his Republican presidential campaign, with top donors warning that the former Florida governor needs to demonstrate growth in the polls over the next month or face serious defections among supporters.

The warnings, expressed by numerous senior GOP fund­raisers in recent days, come as Bush and an allied super PAC are in the early stages of an aggressive television ad campaign they say will help erase doubts about his viability.

But Bush continues to battle against a steady decline in the polls, sinking to fifth place at just 7 percent in a national NBC News/Wall Street Journal poll released Sunday and similarly languishing in the early-voting states of Iowa and New Hampshire.

The warnings from top donors come as Wisconsin Gov. Scott Walker’s exit from the race re­focused the battle within the GOP’s establishment wing as one between Bush and his former protege, Sen. Marco Rubio (R-Fla.). Right now, the momentum appears to be behind Rubio, who has jumped ahead of Bush in most polls. At least a third of the bundlers who signed up to raise money for Walker have switched their allegiance to Rubio, while a smaller number have gone with Bush, according to people familiar with the discussions.

Read the article “Top GOP Fundraisers: It’s Make or Break Time for Jeb Bush” on washingtonpost.com.