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WASHINGTON (Reuters) - Michael Bloomberg, the billionaire media mogul who continues to weigh a run for the Democratic nomination for president, will launch a $100 million online ad campaign targeting Republican President Donald Trump, an advisor confirmed.

Bloomberg has not said whether he will run for president, but has qualified as a candidate to appear on the primary ballots of two states. The moderate former mayor of New York City would have a tough fight to win the primary that began in earnest in the spring and has allowed possible rivals to campaign for months already.

The online advertisements Bloomberg is funding, which have not yet run but are slated to begin Friday, will target four states: Arizona, Michigan, Pennsylvania and Wisconsin. The advertisements are slated to run through the primary season, regardless of whether Bloomberg decides to run for president, the aide confirmed. The details were first reported by The New York Times.

Those four states are considered some of the closest and most hotly contested in the primary contest. Trump won all four in 2016, but public opinion polls have shown that he is vulnerable there in the 2020 election when he will stand for reelection.

Bloomberg pumped millions of his own dollars in the 2018 midterm elections to help Democrats across the country.

The crowded field of Democrats vying for president has largely been focused on the primary contest. Should Bloomberg run, he would join the crowded field, entering tied for fifth place.

Bloomberg’s public flirtation with a run, after saying in March that he would not seek the nomination, came days before former Massachusetts Governor Deval Patrick announced his late entry into the Democratic field.

Democrats have grown increasingly concerned that while their party try to sort out their primary and pick a nominee, Trump will be able to gain ground in contested general election states. The president’s reelection campaign has started spending heavily in online advertising, using the war chest he has already begun to amass to target general election voters.

Bloomberg’s ad buy could serve as a buffer, an effort to offset Trump’s advantage.