Senate Majority Leader Harry ReidHarry ReidWeek ahead: House to revive Yucca Mountain fight Warren builds her brand with 2020 down the road 'Tuesday Group' turncoats must use recess to regroup on ObamaCare MORE (D-Nev.) on Monday moved to hold a vote this week on increasing the federal minimum wage.

Reid filed cloture on the motion to proceed to legislation from Sen. Tom HarkinTom HarkinDistance education: Tumultuous today and yesterday Grassley challenger no stranger to defying odds Clinton ally stands between Sanders and chairmanship dream MORE (D-Iowa) that would increase the minimum wage from $7.25 to $10.10 per hour.

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The vote will be a major test of unity for Senate Democrats, who have made the wage hike central to their populist agenda for the midterm elections. 

Some vulnerable Democrats up for reelection have expressed concern about the wage increase, citing a Congressional Budget Office report that Harkin’s bill would harm job growth. Some of them have come out in support of a smaller increase to $9 an hour, proposed by Sen. Susan CollinsSusan CollinsSchumer: Senate Russia probe moving too slowly Collins: I'm not working with Freedom Caucus chairman on healthcare Mexico: Recent deportations 'a violation' of US immigration rules MORE (R-Maine).

If Democrats were to stick together on the wage vote, they would need at least five Republicans to vote with them to advance the measure. 

But Republicans are unified against it, which has put pressure on Democrats to minimize defections in their 55-member caucus.

Sen. Mark PryorMark PryorMedicaid rollback looms for GOP senators in 2020 Cotton pitches anti-Democrat message to SC delegation Ex-Sen. Kay Hagan joins lobby firm MORE (D-Ark.) has said he’ll vote against the increase to $10.10 per hour, but other vulnerable Democrats up for reelection have signaled they would support it.

Most Democrats are insisting on the $10.10 wage level because they say it would lift all full-time workers out of poverty.