(CNN) A group of senior aides to John Hickenlooper's presidential campaign urged the former Colorado governor to end his bid for the Democratic nomination and run for Senate last month, multiple sources familiar with the discussion tell CNN.

The meeting happened shortly after the Democratic National Committee announced in May that it was increasing the qualification thresholds for the September and October primary debates.

A key pressure point was the fact that Hickenlooper only has 13,000 unique donors, the sources said, making it nearly impossible for him to reach the 130,000-donor threshold needed to qualify for the third and fourth debates.

One source added that Hickenlooper's campaign raised just over $1 million in the second quarter of 2019 and could run out of money soon if no changes are made or if they governor's candidacy doesn't take off.

The meeting between senior aides and Hickenlooper was first reported by Politico

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