Dismayed at China’s tactics in the South China Sea, U.S. senators from both parties on Wednesday demanded the White House show more resolve with Beijing and ratchet up U.S. naval patrols near disputed islands in the strategic waterway.

With President Barack Obama due to travel to Vietnam next month, four senators introduced legislation that calls for bolstering security assistance to allies in Southeast Asia and expanding U.S. military operations meant to uphold the right of freedom of navigation in the South China Sea.

“For too long as China continues its aggressive and expansive policies, the United States has played the role of observer, or perhaps protestor, but not yet actor,” Sen. Robert Menendez (D-N.J.) said in a statement announcing the bill.

The Obama administration has consistently called for China to respect international law and forswear coercion, but to little avail. Last fall, Chinese president Xi Jinping promised in a visit to Washington to halt so-called “militarization” of the region, but instead has only ramped up the dispatch of advanced radars, air defense systems, and even military aircraft to islands claimed by neighboring states.

The broader U.S.-China relationship, which requires cooperation on issues ranging from the global economy to North Korea, makes it hard for Washington to push back too hard on Beijing’s moves in the region — but that tread-softly approach is wearing thin with lawmakers.

At a Senate Foreign Relations Committee hearing Wednesday featuring Deputy Secretary of State Antony Blinken, some lawmakers blasted the administration’s policies on the South China Sea as weak and lackluster.

The Republican chairman of the committee, Sen. Bob Corker of Tennessee, said that “freedom of navigation operations that happen once a quarter are viewed as nothing but symbolic” and that the U.S. Navy should be carrying out the patrols every week or every month.

For years, the United States has routinely sailed through areas it considers part of international waters to assert the principle of freedom of navigation. But the patrols have taken on heightened importance and political weight in the contested South China Sea, where tensions have mounted due to Beijing’s elaborate island-building campaign and its expansionist territorial claims.

The U.S. military has conducted two freedom of navigation patrols near artificial islands or features claimed by China since October 2015. Pentagon officials say the military has been ready to carry out operations more frequently within 12 nautical miles of man-made islands, but the White House so far has chosen to take a more cautious approach.

The proposed legislation is aimed in part at influencing the Obama administration’s policies before the president’s scheduled visit to the region next month, congressional aides said, amid concern among some lawmakers that the United States needs to do more to check China’s assertive moves in the South China Sea.

The bill “encourages them to take a more robust, forward leaning position” and is meant “to put a little steel in their spine,” a Senate staffer, who spoke on condition of anonymity, told Foreign Policy.

The bill “encourages them to take a more robust, forward leaning position” and is meant “to put a little steel in their spine,” a Senate staffer, who spoke on condition of anonymity, told Foreign Policy.The proposed legislation is also designed to send a signal to China about how lawmakers view its aggressive tactics and, as a bipartisan bill, to reassure allies and partners alarmed at some of the more bombastic, isolationist rhetoric in the U.S. presidential campaign.

The proposed legislation, the Asia-Pacific Maritime Security Initiative Act, would require the administration to report to Congress on plans for freedom of navigation operations as well as China’s activities in the disputed waters. The bill also would elevate the status of the Philippines under rules for U.S. security assistance, allowing the country to receive more sophisticated military hardware.

Over the past two years, Beijing has built up a network of artificial islands on disputed reefs and atolls through a massive dredging effort, with runways and deep harbors that can accommodate military aircraft and naval warships.

Sen. Ben Cardin, the ranking Democrat on the Senate Foreign Relations Committee, said the bill was a response to China’s repeated defiance of the rule of law and militarization of the South China Sea.

The senators issued their appeal for a stronger stance as an international court in The Hague weighs a complaint filed by the Philippines that accuses China of violating the laws of the sea with its far-reaching claims in the South China Sea.

With a landmark ruling pending with the Permanent Court of Arbitration, “now is the time for the United States and our regional and global partners to take clear and concrete measures to support a rules-based order for the Asia-Pacific Region,” Cardin said in a statement.

Three Democrats and Republican Sen. Cory Gardner of Colorado sponsored the proposed legislation.

FP senior reporter John Hudson contributed to this article.