NEW DELHI—Prime Minister Narendra Modi led his party to a landslide electoral victory in India’s most-populous state, a result that strengthens his grip on power and lays the groundwork for his widely expected run for a second term in 2019.

The scale of his success shows support for the prime minister wasn’t hurt by his radical decision in November to cancel nearly 90% of India’s paper money overnight. People were forced to line up at banks for weeks to exchange or deposit cash. Mr. Modi said he wanted to punish tax-dodging fat cats.

“It is clear that the people, particularly the poor, are standing with the prime minister like a rock,” Amit Shah, president of Mr. Modi’s Bharatiya Janata Party said in a news conference as the results poured in on Saturday.

The BJP was on course to win 312 out of 403 seats in Uttar Pradesh’s legislative assembly, according to the Election Commission. The numbers indicate widespread support for Mr. Modi across caste divisions that have defined the state’s politics for decades—a development a senior opposition politician, Kapil Sibal, called “astounding” in a televised interview.

In a tweet from his official Twitter account, Mr. Modi said he was “overjoyed that BJP has received unprecedented support from all sections of society.”

With his opponents discredited and broad popular backing for his leadership style, Mr. Modi could use the second half of his term to pursue tougher—and more-contentious—steps to root out corruption and spur development.

Results of other state elections gave Mr. Modi’s party a big win in Uttarakhand over its rival Congress party. The BJP also broke fresh ground with multiple seats in the state of Manipur, where it had won none in the previous election, though it failed to overtake the Congress.

The BJP’s wins will strengthen Mr. Modi’s hand in Parliament’s upper house, where the party doesn’t have a majority. Upper-house members are elected by state legislatures. Opposition parties have obstructed his government’s agenda there.

The stakes for Mr. Modi are even higher. Under his leadership, the BJP strives to amass the kind of ubiquitous political presence the Congress, which led India’s independence movement, commanded in the 1960s and ‘70s. With Uttar Pradesh under its belt, the BJP is one step closer to that goal, now in control of states accounting for more than half of India’s population.

Congress on Saturday won in the state of Punjab, defeating a governing coalition that the BJP was a part of. It also overtook the BJP in the tourist state of Goa. But its crushing loss in Uttar Pradesh, where it had joined forces with another one of Mr. Modi’s rivals in an effort to beat him, plunges the party into a crisis.

Congress was hoping to bring opposition parties together on one platform to halt Mr. Modi’s march in 2019—a plan it will now struggle to implement. The election outcome intensifies a debate over whether its leaders, who belong to the Nehru-Gandhi dynasty, are capable of resurrecting the party’s fortunes.

“At this rate we might as well forget 2019,” Omar Abdullah, an opposition politician, tweeted Saturday, adding Mr. Modi’s rivals were better off planning for national elections in 2024.