INDIAN WELLS, Calif. (Reuters) - The conservative Koch network spent its annual donor conference celebrating policy victories under President Donald Trump such as the tax overhaul, but the elation was tinged with anxiety over November's congressional elections that could pose a risk to its agenda.

To that end, the network plans to spend what would be unprecedented sums for the Kochs to maintain Republican majorities in the U.S. Senate and House of Representatives, while trying to sell voters on the benefits of the newly passed tax package, according to network officials who briefed reporters on their strategy during the conference this weekend in Indian Wells, California.

Historically, the party in power loses seats in congressional elections after a new president's election. This year, Republican angst is compounded by Trump, whose tumultuous presidency helped galvanize Democratic and independent voters to go to the polls in special and state elections last year.

"It's going to be a very challenging environment," said Tim Phillips, president of Americans for Prosperity, a grassroots political group that is part of the Koch network. "The left is energized. There's no question about that."

The network is prepared to spend up to $400 million on the congressional races - a 60 percent increase from its investment in the 2016 election, officials said.

Energy magnates Charles and David Koch have long been outsized players in Republican politics, but they never warmed to Trump during the 2016 campaign.

With Trump in the White House, however, they have seen several policy goals realized, including the tax legislation and cutting federal regulations.