U.S. Democratic presidential candidate Beto O'Rourke speaks at a rally in Los Angeles, California, U.S., April 27, 2019. REUTERS/Lucy Nicholson

LOS ANGELES (Reuters) - Democratic presidential candidate Beto O’Rourke has hired Jeff Berman, one of the architects of Barack Obama’s 2008 presidential victory, as a senior adviser, the campaign said on Thursday.

Berman is an expert in the complex system of how the Democratic Party awards delegates during its presidential nominating contest. His hiring as senior advisor for delegate strategy indicates O’Rourke is digging in for a long delegate battle in a huge Democratic field with 22 candidates so far.

Berman is the second former senior Obama official to join the former Texas congressman’s campaign. Jen O’Malley Dillon became O’Rourke’s campaign manager in March.

“Jeff was one of the first people I reached out to when I came on board because delegate strategy is so critical to our overall strategy,” O’Malley Dillon said in a statement.

Unlike the Republican nominating system, where delegates in most states are awarded on a winner-takes-all system, Democrats award delegates on a proportional basis. That means a candidate does not necessarily have to win individual primary elections and caucuses to amass delegates.

The hiring of O’Malley Dillon and Berman reflects efforts by O’Rourke to bring a more traditional approach to his White House bid after a less structured strategy during his unsuccessful U.S. Senate run last year.

Berman also served as delegate strategist for Hillary Clinton during her 2016 White House run.

O’Rourke attracted an initial surge of publicity and excitement when he entered the presidential race in March. He has struggled in the most recent opinion polls to break out of a second tier of candidates trailing far behind front-runners Joe Biden, the former vice president, and U.S. Senator Bernie Sanders of Vermont.

A CNN poll last week, however, showed O’Rourke to be the leading Democratic candidate to beat Republican President Donald Trump in a head-to-head matchup.