Progressive candidate Abdul El-Sayed held a narrow lead over US Representative Haley Stevens in Michigan's Democratic US Senate primary on Tuesday, though the outcome of a race viewed as a major test of the Democratic Party's future remained uncertain.

With 70% of the estimated vote counted, CNN showed El-Sayed leading Stevens by 49.8% to 46.3%.

The contest has emerged as one of the clearest tests yet of the direction of the Democratic Party, pitting El-Sayed's campaign against Stevens, an establishment-backed moderate supported by Democratic leaders and pro-Israel groups.

The race was being closely watched for clues about whether Democratic voters in a crucial swing state are receptive to candidates who challenge party leaders and champion an agenda focused on lowering costs, expanding healthcare coverage and restricting the role of big money in elections.

The winner will face Republican Mike Rogers in November in a race critical to Democrats' hopes of regaining control of the US Senate.

Battle between progressives and moderates

The Democratic race is the latest battleground between progressives and moderates over US support for Israel, an issue that has played out in primaries nationwide this year. But unlike contests in liberal bastions, this is a statewide race in a Midwestern swing state for a seat Democrats must win in the fight for Senate control.

Michigan is both a presidential battleground and home to one of the country's largest Arab American communities, making it an important testing ground for Democrats' electoral and ideological future.